OLED's are preferred as display devices because each OLED emits light by itself and backlighting is not required. Therefore, in today's rapidly advancing communication and video display technologies, it is becoming more and more desirable to utilize TFT-OLED (Thin Film Transistor-Organic Light Emitting Diode) arrays as display devices in various applications including those in which the display device is of considerably larger size. A challenge in providing OLED arrays of increased dimension, however, is that the power line and control signal lines necessary to operate the display devices of large dimensions include correspondingly increased power and current levels. When conventional technology is used to form the power and control signal lines of increased power/current levels, they are susceptible to blow out.
Power lines and control signal lines are conventionally formed on a printed circuit board that is coupled to the substrate including the TFT-OLED array. The printed circuit board is advantageously coupled to the substrate by use of an anisotropic conductive film (ACF) that bonds the printed circuit board to the substrate. A method for mounting a flexible printed circuit board to a substrate using an anisotropic conductive film is disclosed in commonly assigned pending U.S. patent application Ser. No. 10/766,536, filed Jan. 27, 2004. The anisotropic conductive film includes a plurality of conductive spheres dispersed in an adhesive resin. When the ACF is interposed between a printed circuit board and substrate and a compressive force is applied, the conductive spheres rupture, providing electrical connection. Typically, the printed circuit board includes a number of terminals associated with conductive lines formed on the printed circuit board and the terminals are electrically coupled to associated contacts on the substrate through the ACF. The contacts may be terminals of interconnect leads that are coupled to the TFT-OLED array. Another shortcoming associated with using conventional technology to form display devices including large OLED arrays, is that when a control signal line and power line are coupled to a substrate through an ACF and the necessarily high current is applied, the resulting high temperatures can cause the ACF to break down and undesirably short together terminals and contacts that are in close proximity to one another.
A prior art TFT-OLED arrangement is shown in FIG. 1. FIG. 1 shows TFT-OLED array 100 formed on substrate 102. The OLED's may be bottom-emitting OLED's, i.e., they emit light through substrate 102, or they may be top-emitting OLED's, i.e., they emit in the opposite direction. Signal line 104 and OLED power line 106 are formed on flexible printed circuit board 108 which is coupled to substrate 102. Frame 110 will also be coupled to substrate 102. Signal line 104 and OLED power line 106 may be formed of conventional materials such as copper leads formed on flexible printed circuit board 108. FIG. 2 shows an exploded side view of the prior art display device also shown in FIG. 1. Conductive lines 104 and 106 of flexible printed circuit board 108 are coupled to substrate 102 through ACF 112. Each of signal line 104 and OLED power line 106 typically terminate at a plurality of terminals that are electrically coupled to a corresponding plurality of contacts on substrate 102. As shown in FIG. 1, conductive signal line 104 and conductive power line 106 are connected to about the same peripheral location on substrate 102. When the size of TFT-OLED array 100 is large, signal line 104 and OLED power line 106 must accommodate high currents. Over the lifetime of the display device, the high currents cause either the conductive lines (104 and 106) themselves to break down or produce high temperatures which causes ACF 112 to break down and undesirably cause the various terminals and contacts associated with signal line 104 and OLED power line 106 to short. A typical conductive line that may be used as OLED power line 106 or signal line 104 may be a copper interconnect lead having a width of about 10 mils or 0.254 mm. Such a copper interconnect may safely accommodate currents in the vicinity of 500 milliamps. A 2″ by 2″ TFT-OLED array may utilize a current of 1 to 2 amps in “bright mode” when all OLED's are illuminated, however. Larger arrays have correspondingly higher current levels and it can be seen that even for a 2″×2″ array, conventional copper interconnect leads are incapable of accommodating the associated current and are subject to being blown out and heating up causing the degradation and destruction of the ACF.
It would therefore be desirable to provide a display device that includes the advantage of OLED technology, the advantage of an anisotropic conductive film to connect current carrying lines to the substrate that includes the OLED array. It would be further desirable to provide a display device of increased dimension and which can accommodate the large currents necessary to power a large size array, without fear of the conductive lines being blown out or shorted due to the breakdown of the anisotropic conductive film.